Different output image when I set "use_borders = True" - in python script
It is caused (probably) by compositor. If I render part of image, compositor change only this one piece (tile) - without affecting another tiles. After joining them, seams can be seen.

To reduce seams I will render scene by scene (without using compositor - I will set bpy.context.scene.render.use_compositing = False), tile by tile (saved to erx-multilayer format) and then join outputs. Then I will change inputs in compositor (render layers will be replaced by images).
I hope it will work.